Sessions recorded in previous versions of Ardour (always downloaded from ardour.org), notes are sometimes offset within a MIDI region.

The region is in the correct location but the notes within the region are offset from where they should be and go over the end of the region boundary (highlighted in red in the attached screenshot). This happens with some regions in some sessions, but not all the time -- I've just seen a second occurrence of it (with the first occurrence I wasn't sure it wasn't user error and no longer had A5.8 in order to test). In one case, I think not only were the notes offset but the tempo of the region was wrong.

The blue highlights show where notes are offset from where they should appear. On the track *Hmd the notes should start at the beginning of bar 57 and on the track *T-3V the notes should start at bar 87 (where the meter changes to 10/8).

Not only are the regions visibly wrong/offset, but often the MIDI notes don't play or only play intermittently such as when the track is solo'd or the plugin muted then unmuted.